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Is your unoptainium power switch worth $5 and an hour or so of your time?

Discussion in 'Solid State' started by K7sparky, Feb 26, 2013.

  1. K7sparky

    K7sparky AK Subscriber Subscriber

    Messages:
    237
    Location:
    Great State of West TN
    I finally got around to trying a Triac to turn on and off an amp and it was a success. I am still looking for the down side of this mod.

    This mod should work on any amp and the power switch load will be reduced to around 120 ma and no arcing. The power switch will now last longer that you will.

    The test bed is a Lafayette LR9090 (Setton RS) series known for lunching on power switches. This switch had the NO contact for power "on" inside the switch gone, just smoky residue left. This special case required a relay NC contact for the PA Mute / protective relay dump on power "off". I moved the switch NC contact to NO position for the power on function.

    Triac used was a Littlefuse Q4025L6 (Mouser 576-Q4025L6) Rated 400VAC 25A RMS for highly inductive loads and tab isolated. Way over rated. Yea olde Fluke gives around 7A AC peak on the 100 ms scale at power "on" and idle current of 0.6A AC at low volume. The Triac data sheet is on Mouser: http://www.mouser.com/Search/Refine.aspx?Keyword=q4025

    Note: Triac suffix L6 is a TO220 case isolated tab, if you prefer solder loops for a buck or so more the J6 suffix is a TO218X with loop leads and isolated tab.

    For me, the isolated tab and not having to bother with mica and screw isolator is worth a buck or 2. You?

    This is a difficult mod with an extensive parts list:
    1 ea Triac
    1 ea 100 ohm 1/2 watt resistor

    Remove the AC line power and transformer lead (and maybe switched rear outlet) from the power switch. Remove the RC snubber across the switch contacts.

    Mount the Triac in any handy location.
    AC power lead goes to M1
    Transformer lead (and maybe switched rear outlet) goes to M2
    Power switch:
    lead 1 100 Ohm resistor and goes to gate G
    lead 2 goes to M2

    [​IMG]

    OK!, so I lied about being difficult

    I found a place on the chassis for the Triac that the leads would fit, drilled a mounting hole, soldered directly to the TO220 leads and used heat shrink over joint and lead.

    Initial testing was done by removing the transformer primary fuse and hooking up a 100W bulb for a load. After a bunch of switching the bulb on and off, I put the transformer fuse back in and tested and tested...... The amp has been switched on & off left on for days and no problems.

    On state voltage drop is 0.8 to 0.9 VAC which agrees with the data sheet curve

    Other benefits / thoughts:
    1. Littlefuse calls these Triacs: Alternistor Triacs and they are designed for high inductive loads so no RC snubber is required.
    2. Power "off" is always at a zero crossing so there is no arcing thus no voltage transients inside the amp to worry the power supply components.
    3. Power "on" has no contact bounce arcing. It did not seem useful to add the extra parts for zero crossing turn on.
    4. A 200V rated Triac would work, but why save a few pennies and give Murhpy a chance?
    5. Lower current rating would work. 3000W @ 125 VAC switching capability is not needed, but see above
    6. Triac off state leakage is 50 micro amps which is lower than the original RC filter across the power switch.
    7. A MOV and / or RC filter on the incoming power is still necessary to protect the amp from the rest of the world.

    After more testing this mod will be refined with pictures and added to the LR9090 repair / upgrade thread.

    I hope to get some feed back about possible problems I have missed.
     
    krlill likes this.
  2. thefragger

    thefragger Certified Crazy.

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    Location:
    Toronto, Canada
    Exciting stuff, thanks for bringing this to the table!
     
  3. goodolpg

    goodolpg AK Member Subscriber

    Having just gone through power switch problems (as you know) I wish I had the abilities to use your fix.
    Hopefully others will be able to.
    Thanks for the thread.
     
  4. Son-of-Vere

    Son-of-Vere Super Member

    Messages:
    1,526
    Location:
    The Land that Audio Forgot
    Looks like something I could use, I hear a lot of arcing when powering up my Marantz stuff.
    Do you have any specs for noise? I always thought triacs created some annoying buzzing. (They're used in dimmers!).
    Maybe a scope pattern?
     
  5. EchoWars

    EchoWars Hiding in Honduras

    Messages:
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    Location:
    Kansas City
    Well, they're gonna get a tad warm. I could not find a graph of M1/M2 voltage vs. current. but the dissipation vs current curve indicates that the M1/M2 voltage remains fairly constant. What that means is that you can figure 1W of dissipation for every amp of conduction. On a large high-power receiver this can be substantial.
    They're noisy in dimmers because the conduction angle is being varied and the device is being switched on and off at 'high' currents. Used as a simple power switch they should be conducting at a full 360° and thus quiet as a mouse.
     
    Last edited: Feb 26, 2013
  6. Jon_Logan

    Jon_Logan Addicted Member

    Messages:
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    Location:
    Left Coast
    Don't you want a snubber across your TRIAC (T1-T2) also?
     
  7. K7sparky

    K7sparky AK Subscriber Subscriber

    Messages:
    237
    Location:
    Great State of West TN
    Jon Logan: No snubber required.
    Littlefuse (subsidiary Teccor) rates these for high inductive loads and states on the data sheet "no snubber required"

    The scope trace is a plane 60 cycle sin wave. The 1 volt center step is not visible that is .8%. I will shoot a pix of the scope trace tomorrow and see if it turns out and post it. As Echowars pointed out i hear nothing. The Transformer heating was what I wondered about and that seems a non issue so far.

    As far as transmitted / radiated noise I can not detect any change amp on or off on any of the ham bands 10M to 160M. Good idea i will fire up the spectrum analyzer tomorrow and sniff around with it.

    I have 2 LR-9090 one with Triac and one with an ice cube relay to save the power switch on and shooting the transformers with a temp gun they are both 98F after about 40 hours at moderate volume.

    The transformer and heatsinks have heated the chassis to 79F and the Triac is 84F as close as I can shoot it. The chassis of the amp without the Triac is the same temp.

    Echowars: the data sheet E4 from Mouser page 7 figure E4.7 curve for 25A to 35A devices shows about 1.25 V M1 to M2 at 25A or 3000 watts controlled power. That is dissipation of 31W controlling 3KW @ 120 VAC if I am reading it correctly and it agrees with your 1W per controlled amp at line voltage VAC. I am not running any of my audio amps on 220 VAC.

    Thanks for the comments so far. This still seems to good to be true.
     
  8. Jon_Logan

    Jon_Logan Addicted Member

    Messages:
    6,881
    Location:
    Left Coast
    Crown had done the switch (har-har) from a power relay to a SSR (Solid State Relay) during their M-600 series run. It is a tried and true upgrade. I remember the noise performance on the SSR loaded amp to be very good. So whatever Crown did, the amp stayed quiet. The voltage across the TRIAC is low because they turn on shortly after passing through 0 V.


    A heatsink is better, but not a big issue.
     
  9. glen65

    glen65 Member

    Messages:
    4,276
    Location:
    Richland county, Ohio
    No, what youre doing here is quite sound.
    While many relays are used, triacs have been used quite a bit in the
    appliance industry. Many of the Microwave ovens I've serviced used triacs
    to switch on the primary of the high voltage transformer. The most simple
    of the lot, use one of either below with an external snubber. Not that you
    need it here, but you could add a small sensitive gate triac in the
    mix driving the other, which would bring down the current across the
    switch contacts down into the 10s of milliamps or less. If it were needed,
    another device that could be used to handle the transients would be a
    bidirectional diode. But apparently such details have been taken care
    given the device you've chosen.

    [​IMG]

    [​IMG]
     
    Last edited: Feb 26, 2013
  10. EchoWars

    EchoWars Hiding in Honduras

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    Location:
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    I'd like to see this. But...consider that we're sending this to the transformer secondary and then through a bridge and onto the PS caps, which are charged at the peaks as opposed to anywhere near zero crossing anyway, so I'm not convinced that the triac can have an effect on the available DC one way or another.
    Please describe your concerns...I'm not sure I follow.
    I'd look again, except that the 1W per amp seems to be a pretty good rule of thumb here. Like I said, in a big high-powered unit, heatsinking is a concern.
     
    Vinylcafe likes this.
  11. so_ein_pech

    so_ein_pech toob noob

    Messages:
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    Location:
    Somewhere near Austin, TX
    This is a cool mod and it seems almost universally applicable. I want to try it out on one of those rotary Alps switches that are prone to arcing (like in many pioneer SX-x50's and some Kenny's).

    Do you know if it also solves the speaker popping problem that you get on some amps when powering down? Is that why you say it doesn't need a snubber cap?
     
  12. mech986

    mech986 Text ↓ optional Posts:>18,000

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    Interesting mod and simpler than the switch activating a power relay. what about the switch activating a solid state relay, assuming there's room for one? I've always thought that would be the way to go.
     
  13. gridleakrick

    gridleakrick Well-Known Member

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    San Diego, CA
    The triac IS a solid state relay.
    Rick
     
  14. peterh

    peterh AK Subscriber Subscriber

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  15. SPL db

    SPL db It's all about the music! Subscriber

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    Can you reuse the bad power switch in it's current state or does it still need to be replaced and this mod makes the new one last that much longer? :scratch2:
     
  16. thefragger

    thefragger Certified Crazy.

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    Location:
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    You still need a functional switch, however the working current across the contacts is cut significantly.
     
  17. K7sparky

    K7sparky AK Subscriber Subscriber

    Messages:
    237
    Location:
    Great State of West TN
    SPL db Last question first: a bad switch is a bad switch. However you might get it cleaned up / repaired enough to carry .2A to a resistive load where it would never handle a transformer inrush. It needs to be a good low resistive contact. Depends on what you have left of the switch and how good you are at repairing things. Also as Glen65 points out you can add a sensitive Triac to fire the line Triac reducing switch current to a few ma.

    mech986: I too started with thoughts of a SSR. Takes more room and costs a LOT more for no improvement I could see.

    I used the isolated tab TO 220 package so I could just screw it to the chassis and have the heat sink problem done with. Takes up almost no room at all.

    [​IMG]

    If you look at the SX 1980 specs and take the CSA power spec of max 1400W max, that would be Triac dissipation of about 12W. My LR 9090 NP is AC 240W or about 2W Triac dissipation.

    The transformer heating concern was from an old experience with a SCR switching a transformer input for voltage control and caused a bit too much heating, but so long ago I forget the details.

    I also forgot that I have been using a plain vanilla TIC265m 600V 25A Triac as a front end regulator on an OLD BIG 220VAC input transformer for a 3KV plate supply for a pair of 3-500Z. That has been running for over 25 years with no issue. That one has a MOV across the Triac and is opto isolated from the 3KV it is regulating. The transformer makes some audible noise at 2KW into a dummy load but lasted through many 48 hour contests at legal limit. It has never caused any RFI that I have detected.

    I thought I had shot scope pictures with a digital camera OK, but it is a bit tricky. The screen is fine to the eye, but the CCD camera doesn't get the full trace. With storage on it is really blurry so I shot a bunch and took ones with the most of the trace on the screen.

    Probes are 10X, Add 1&2, Invert 2, sweep 5ms, line trigger. Probes adjusted on square wave internal calibrator. Sony Tek storage scope ground floated.

    Line voltage was about 121 VAC

    [​IMG]

    Across Triac - inputs set at .2V / div (Fluke 87 measures 1.047VAC for what it's worth)

    [​IMG]

    Across transformer primary - inputs set at 10V / div. (121 * 2 / .707 = 342V PP)

    so ein pech: Does this LR Alps SW look like the SX & Kenny one you mention? This one I pulled apart, cleaned up with scrubbing bubbles and moved the NC contact for mute to NO for power on and use an ice cube relay for the mute function. There was no contact left on the power side when I opened it up.

    [​IMG]
     
  18. Son-of-Vere

    Son-of-Vere Super Member

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    Hey thanks for the extra effort on the scope pics, looks like a winner to me!
     
  19. so_ein_pech

    so_ein_pech toob noob

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    Yup. I had one arcing on an SX550. I was able to burnish it enough with a little sandpaper to stop the arcing. But its probably a temporary fix.
     
  20. K7sparky

    K7sparky AK Subscriber Subscriber

    Messages:
    237
    Location:
    Great State of West TN
    so ein pech:
    Wait a bit more to see if anyone can find a problem with this mod (my LR 9090 - 90W / ch continuous full spectrum both channels driven - with the same switch is a happy camper) and then:

    Clean up the inside of the switch with scrubbing bubbles and a tooth brush. The one in the picture was BLACK inside when I opened it. The bubbles take all the carbon / soot off the plastic and contacts. The rough contact surface is a GOOD thing. The tiny arc points make the initial contact much better and faster then a smooth contact surface.

    Check contacts with an ohmmeter after the cleaning and only if necessary then a LIGHT burnish with a burnish tool or around 600 grit paper.

    A TINY bit of lube on the black rotor.

    Do the Triac mod so you have a resistive 120 ma contact load and the switch will outlast you and no more arcing. Don't forget to remove the RC filter across the switch contacts.

    I had been installing a small ice cube relay for the switch to operate after cleaning it up. The relay switched the transformer, rear switched outlets and a NC for the PA mute if I had to rob the NC switch contacts because the NO ones are burned up.

    From now on a relay only if a NC function is needed & a Triac for transformer / switched outlet duty.
     

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